This invention relates to an electronic circuit for providing a linear output from an amplitude modulated transmission device such as a semiconductor laser which has an output distorted from its input due to inherent nonlinearity. The distortion of the nonlinear device is compensated by applying a predistorted signal to the input of the nonlinear device. The predistortion is chosen such that the distortion of the nonlinear device restores the undistorted signal.
Directly modulating the analog intensity of a light emitting diode (LED) or semiconductor laser with an electrical signal is considered among the simplest methods known in the art for transmitting analog signals, such as sound and video signals, on optical fibers. Although such analog techniques have the advantage of significantly smaller bandwidth requirements than digital pulse code modulation, or analog or pulse frequency modulation, amplitude modulation may suffer from noise and non-linearity of the optical source.
Distortion inherent in certain analog transmitters prevents a linear electrical modulation signal from being converted linearly to an optical signal, and instead causes the signal to become distorted. These effects are particularly detrimental to multi-channel video transmission which requires excellent linearity to prevent channels from interfering with each other. A highly linearized analog optical system has wide application in commercial TV transmission, CATV, interactive TV, and video telephone transmission.
One method employed in the past to reduce distortion inherent in nonlinear devices has been predistortion. In this technique, a modulation signal is combined with a signal equal in magnitude to the distortion inherent in the nonlinear device but opposite in sign. When the nonlinear device modulates the combined signal, the device's inherent distortion is cancelled by the combined signal's predistortion and only the linear part of the source signal is transmitted.
As described in the parent application, a predistortion technique divides an input signal into two or more electrical paths and generates predistortion on one or more of the paths resembling the distortion inherent in the nonlinear transmitting device. The generated predistortion is the inverse of the nonlinear device's inherent distortion and serves to cancel the effect of the device's inherent distortion when recombined with the input signal.
Attenuation is used to match the magnitude of the predistortion to the magnitude of the device's inherent distortion characteristics before the signals are recombined and sent to the nonlinear device for modulation.
The method compensates for the distortion of nonlinear devices in which the amplitude and phase of the distortion depends on the frequency of the distortion signal.
An additional effect of frequency may appear in some nonlinear devices. Some nonlinear devices exhibit the characteristic that the distortion generated depends on the frequencies of the fundamental signals in addition to the previously described dependence on the frequency of the distortion signal. As an example, a device with a second order nonlinearity may generate distortion at 50 MHz due to fundamental signals at 100 MHz and 150 MHz, or due to fundamental signals at 500 MHz and 550 MHz. In some nonlinear devices, the distortion generated by these two pairs of fundamental frequencies can be substantially different. If this is the case, the circuit described in the parent application may not adequately correct for the nonlinear distortion signal at 50 MHz.
In practice of this invention, a filter is included in a secondary path preceding the distorter so that devices exhibiting distortion that is dependent on the frequencies of the fundamental can be linearized in a substantially more effective manner.
It may also be desirable to change the predistortion as the laser or the predistortion circuitry ages and changes characteristics. Preferably, this is done automatically.
As predistortion circuits have become more sophisticated, additional refinements for greater linearity and use for other applications have been recognized. The present invention accordingly is addressed to these and other desirable features.